Conventional optical navigation systems, such as an optical mouse for a computer, collect light reflected at a specular reflection angle. Highly reflective surfaces do not scatter very much light and can produce a bright (i.e., high intensity) image. When used on highly polished or glossy surfaces (e.g., glass), the signal intensity of the reflected light is greater than that of a non-glossy surface. However, the contrast of the image is low because of the high intensity of the specular reflection signal. The relatively low contrast makes it difficult to distinguish individual features on the surface and, hence, makes it difficult to determine movements of the device based on relative movements of the features in sequential navigation images.
FIG. 1 illustrates an arrangement for a conventional optical navigation device 10. The conventional optical navigation device 10 includes a light source 12 and an image sensor 14. The light source 12 directs light toward a reflective navigation surface 16. The incident light approaches the reflective navigation surface 16 at an angle of incidence. The light reflects off of the reflective navigation surface 16 at an angle of reflection, which is equal to the angle of incidence, because of the reflective nature of the reflective navigation surface 16. The image sensor 14 receives the reflected light and, over time, generates a sequence of images of the reflective navigation surface 16. The conventional optical navigation device 10 also includes an imaging lens 18 to more or less image the illuminated area on the navigation surface onto the image sensor 14.
Due to the specular nature of the reflective navigation surface 16, some of the light that is incident on the reflective navigation surface 16 is specularly reflected toward the image sensor 14. Also, the specularly reflected light does not exhibit a significant pattern because of the uniformity (i.e., low contrast) of the reflective navigation surface 16. Therefore, the high intensity of the specular reflection and the low contrast of the resulting image based on that reflected light render relatively little detail for use in determining movements of the conventional optical navigation device 10. This lack of detail in the resulting images makes it difficult to accurately determine the movements of the conventional optical navigation device 10 relative to the reflective navigation surface 16.